The invention relates to a method and a device for recovering metals from a metal-containing flow, for example from ores, metal-containing residues and waste materials.
With known methods and devices for recovering metals from metal-containing flows, a great of energy is consumed. Said metal-containing flows may be flows of solid materials, for example ores, or of liquid materials, for example metal-containing slurries or solutions. Since the energy is usually generated by the combustion of fossil fuels, large amounts of gases, such as carbon dioxide, are emitted. The emission of such xe2x80x9cgreenhousexe2x80x9d gases affects the environment. Moreover, due to the large energy consumption, such methods and devices are not very efficient.
The invention provides a solution for the above problem. Accordingly, the invention relates to a method as referred to in the introduction, wherein:
a. the metal-containing flow and a solvent are supplied to a dissolving unit, whereby a metal-containing solution is formed;
b. the metal-containing solution is then supplied to a concentration unit;
c. the metal-containing solution is separated in the concentration unit into a small-volume flow containing a high concentration of metal salts and/or metal hydroxides, and a large-volume flow containing a low concentration of metal salts and/or metal hydroxides;
d. the small-volume flow containing a high concentration of metal salts and/or metal hydroxides is supplied to an electrochemical unit; and
e. the small-volume flow containing a high concentration of metal salts and/or metal hydroxides is separated in said electrochemical unit into a flow containing one or more metals, and a flow containing a low concentration of metal salts and/or metal hydroxides.
The term metal salts and/or metal hydroxides is understood to include inorganic and metallo-organic compounds containing metal cations, wherein said compounds may be single salts (for example nickel (II) chloride) or complex salts (that is, salts which contain the same metals exhibiting different stages of oxidation, or different metals, which may or may not exhibit the same stage of oxidation, or single hydroxides (for example tin hydroxide) or complex hydroxides (for example borates), and wherein one or more ligands, for example ammonia, may bonded to the metal cations.
The advantages of the invention are that the method requires much less energy than usual, that the method proceeds quickly and that it is possible to use devices of much smaller dimensions than those which are used with methods according to the prior art. These advantages are achieved in particular because separate circuits are used for the dissolving method and for the electrochemical method. Furthermore it is possible to carry out parts of the methods or the entire method as such for recovering all types of metals, in particular the metals copper, lead, tin, zinc, antimony, chromium, gold, cadmium, silver and nickel, and alloys such as brass. Furthermore it is possible to adapt method parameters such as the flow rate, the temperature and the like to the demand and to the chemical and/or physical requirements which are made of the raw material and of the final product.
In the dissolving unit, a metal-containing flow, for example scrap, sludge and/or ash, is mixed with one or more solvents, for example water. The solvents may contain additives, which promote the dissolution of the metals. Examples of such additives are acids and bases. When the metals are being dissolved, a solution of metal salts and/or metal hydroxides is formed, as well as a vapour and gas flow, which substantially contains solvent in the vapour phase, for example water vapour, and reaction gases, for example hydrogen, carbon dioxide, ammonia, oxygen, nitrogen and nitrogen oxides.
The separation of the solution in the concentration unit preferably takes place by cooling down the solution, whereby a heat-containing flow and a cooled-down solution are formed. Part of the heat which is contained in the original solution is transferred to the heat-containing flow as a result of the evaporation of volatile components which are present in the solution, so that the original solution is cooled down. The heat is preferably transferred as a result of the evaporation of the solvent or the solvents, for example water or a mixture of water and one or more other solvents, from the solution. Accordingly, the heat-containing flow substantially consists of a vapour-phase solvent or solvents from the solution, and in particular substantially of water vapour.
The heat-containing flow is preferably led to a heat exchange plant, so that the heat of the heat-containing flow can be transferred to another process flow, which is to be heated. Consequently, the heat is largely withdrawn from the heat-containing flow, so that the vapour-phase solvent or solvents will condense and a relatively cold flow consisting of a solvent or solvents is formed. Said relatively cold flow consisting of a solvent or solvents can then be led back to the concentration unit.
When the solution is cooled down to a temperature below the saturation temperature, metal salts and/or metal hydroxides will separate from the solution, for example by precipitating or crystallizing. According to the invention the cooled-down solution is thus carried to a settling unit, where the cooled-down solution is separated into a small-volume flow containing a high concentration of metal salts and/or metal hydroxides, and a large-volume flow containing a low concentration of metal salts and/or metal hydroxides.
The large-volume flow, which contains a low concentration of metal salts and/or metal hydroxides, can be led back again, for example to the dissolving unit or to the concentration unit. According to the invention, this large-volume flow, which contains a low concentration of metal salts and/or metal hydroxides, is preferably led back to the dissolving unit.
The large-volume flow containing the low concentration of metal salts and/or metal hydroxides is preferably heated before being led back to the dissolving unit or to the concentration unit. Preferably, the heat which can be transferred via the heat-containing flow and the heat exchange plant is used for heating said large-volume flow. A major advantage of this embodiment is the fact that the energy requirement of this method is much lower than that of comparable, conventional methods.
The small-volume flow containing a high concentration of metal salts and/or metal hydroxides is fed to an electrochemical unit. The small-volume flow containing a high concentration of metal salts and/or metal hydroxides is preferably heated before being led to the electrochemical unit, whereby the required heat may be withdrawn from the heat-containing flow. The required heat may also be provided by using a heat source.
In the electrochemical unit, the small-volume flow containing a high concentration of metal salts and/or metal hydroxides is separated into a flow containing one or more metals and a flow containing a low concentration of metal salts and/or metal hydroxides.
It is preferred to lead back part of the flow containing a low concentration of metal salts and/or metal hydroxides to the concentration unit, whilst another part is led back to the electrochemical unit. More in particular, only a small part of the flow is led back to the concentration unit, whilst the larger part of said flow is led back to the electrochemical unit. According to the invention, it is generally not necessary to heat the flow containing a low concentration of metal salts and/or metal hydroxides that is led back to the electrochemical unit. If it should be necessary to heat said flow, however, this will be possible, of course, for example by mixing said flow with the small-volume flow containing a high concentration of metal salts and/or metal hydroxides prior to heating the latter flow.
The method according to the invention may be carried out in batches or in a continuous process. It is preferred to carry out the method in a continuous process.
The invention furthermore relates to a device for recovering metals from a metal-containing flow, which device comprises a dissolving unit, a concentration unit, and an electrochemical unit.
The concentration unit preferably comprises a heat exchange plant and a settling unit. Said heat exchange plant is preferably provided for transferring heat from the solution of the metal salts and/or metal hydroxides, via a heat-containing flow, to another process flow that is to be heated, preferably the flow from the concentration unit, which contains a low concentration of metal salts and/or metal hydroxides, which flow is preferably led back to the dissolving unit. It is preferred to provide a settling unit for separating metals from the solution which has been cooled down in the heat exchange plant, whereby forming a small-volume flow containing a high concentration of metal salts and/or metal hydroxides and a large-volume flow containing a low concentration of metal salts and/or metal hydroxides are formed.
The heat exchange plant preferably comprises one or more evaporation units for evaporating the solvent or the solvents which are present in the solution, and one or more condensation units for condensing the vapour-phase solvent or solvents.
According to the invention, the evaporation unit preferably operates at low pressure, and the condensation unit preferably operates at high pressure. The combination of evaporation unit and condensation unit, or the combinations of various evaporation units and condensation units preferably operate via the socalled heat pump principle.